quanta: layer.conf: Add QUANTABASE

Define a variable that recipes can use for finding the root
of the quanta layer.  Typically this is used to refer to the
layer LICENSE for recipes that don't have any source repository.

(From meta-quanta rev: 0ac2ffc05a27ca1e880c7f95e86be7241c721c38)

Change-Id: Ia9d9917525eb6197b9acd957e57784c2fb8d6c14
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>